Merge pull request #12827 from hrnr:stitching_4

[evolution] Stitching for OpenCV 4.0

* stitching: wrap Stitcher::create for bindings

* provide method for consistent stitcher usage across languages

* samples: add python stitching sample

* port cpp stitching sample to python

* stitching: consolidate Stitcher create methods

* remove Stitcher::createDefault, it returns Stitcher, not Ptr<Stitcher> -> inconsistent API
* deprecate cv::createStitcher and cv::createStitcherScans in favor of Stitcher::create

* stitching: avoid anonymous enum in Stitcher

* ORIG_RESOL should be double
* add documentatiton

* stitching: improve documentation in Stitcher

* stitching: expose estimator in Stitcher

* remove ABI hack

* stitching: drop try_use_gpu flag

* OCL will be used automatically through T-API in OCL-enable paths
* CUDA won't be used unless user sets CUDA-enabled classes manually

* stitching: drop FeaturesFinder

* use Feature2D instead of FeaturesFinder
* interoperability with features2d module
* detach from dependency on xfeatures2d

* features2d: fix compute and detect to work with UMat vectors

* correctly pass UMats as UMats to allow OCL paths
* support vector of UMats as output arg

* stitching: use nearest interpolation for resizing masks

* fix warnings

modules/python/test/test_stitching.py

@@ -10,7 +10,7 @@ class stitching_test(NewOpenCVTests):
         img1 = self.get_sample('stitching/a1.png')
         img2 = self.get_sample('stitching/a2.png')
 
-        stitcher = cv.createStitcher(False)
+        stitcher = cv.Stitcher.create(cv.Stitcher_PANORAMA)
         (_result, pano) = stitcher.stitch((img1, img2))
 
         #cv.imshow("pano", pano)